This interdisciplinary project has as its objective the elucidation of the molecular mechanisms by which aminoglycoside antibiotics cause permanent hearing loss. It begins with the hypothesis that these antibodies bind to polyphosphoinositide lipids, displace calcium and disturb membrane structure and function. The hypothesis will be tested by a combined in situ/in vitro approach. In situ, aminoglycosidic antibiotics, fragments and derivatives will be investigated by cochlear perfusion in the guinea pig for their effect on cochlear microphonic potentials and polyphosphoinositide labeling. In vitro studies with monomolecular films and liposomes of polyphosphoinositides will focus on drug/calcium interactions, on the stoichometry of the drug/lipid complex and on the determination of those sites on the antibiotic which are involved in its ototoxic action. This system will also be investigated for its usefulness as an in vitro correlate of drug ototoxicity. Another clinically important aspect of aminoglycoside action is the ototoxic interaction of these drugs and other ototoxic compounds. We hypothesize that the aminoglycosides, as a consequence of their membrane action, lower permeability barriers for other drugs, increasing their levels in the cochlea. This hypothesis will be tested by measurements of cochlear microphonics and of cochlear drug levels after combined treatment.